The present invention generally relates to a surface light source panel adapted to guide and illuminate an incident light from a light source provided at an end of the panel for use in various kinds of illuminating devices such as an illumination signboard, display panel or backlight of liquid crystal display, etc., and more particularly to a surface light source panel representing improved brightness and uniformity of light.
Generally, in the surface light source panel of the type referred to above, it is relatively bright in the vicinity of the light source at an end portion of the panel, but, the incident light is difficult to be guided well to a position of the panel far from the light source. Therefore, it has been regarded extremely difficult to realize a surface light source panel having uniformity in brightness. Many ideas intended for improvement have been proposed, though.
For example, Japanese Patent Laid-open Application Nos. 60-87387, 60-181780, 62-902, etc. have been proposed to laminate transparent resin substrates to constitute a surface light source panel, each of which substrate is provided with a means for generating a diffused or irregular reflection so that the light guiding efficiency of the panel can be secured. Japanese Patent Laid-open Application No. 57-128383 has disclosed a surface light source panel in which sections for generating diffused or irregular reflection are formed on a transparent resin substrate by, e.g., the coating of the paint of white family in such a manner that the area ratio of the sections becomes sequentially larger from a position of the light source at an end portion of the substrate, whereby the light guiding efficiency of the panel is improved. Furthermore, it has been proposed in Japanese Patent Laid-open Application No. 60-205576 to improve the light guiding efficiency of a surface light source panel so that a totally uniform section in which diffused reflection is produced is provided in a transparent resin substrate by a coating of a white ink containing glass beads.
The light guiding efficiency of the surface light source panel may possibly be improved to some extent by these proposals as mentioned above. However, any of the proposed surface light source panels is hardly said to be practical in terms of the brightness and uniformity of light. Especially, the difference is conspicuous when comparing a signboard using any one of the proposed surface light source panels with an illumination signboard which has many fluorescent lamps, etc. provided at the back thereof. The signboard using the proposed surface light source panel tends to be illuminated only in the vicinity of the light source, since the light guiding efficiency of the panel is not sufficient. The maximum brightness is generally 300 nit (cd/m.sup.2) or so in the case of the illumination by a light source at one end portion of the panel. Consequently, the signboard using the conventionally-proposed surface light source panel has a level of brightness that gives a considerably dark impression as compared with the backlighting illumination signboard. The brightness of the surface light source panel has been thus recognized to be such as referred to above, at the most.
Although the brightness and the uniformity of light by the surface light source panel are required depending on the useful purpose and the like, the improvement should be urgent so that the surface light source panel is able to be widely used in various ways.